1. Field of the Invention
The invention pertains to an imaging installation, especially for radiology, comprising a television camera with a pick-up device presenting no remanence or low remanence.
2. Description of the Prior Art
A radioscopic installation generally comprises a television camera for the viewing of images given by a luminance amplifier that transforms an X-ray image into an image in visible light. The camera can be used to view images on a television-type screen. In a so-called "digital" radiology installation, the analog signals given by the camera are converted into digital signals, thus making it easier to process the signals, especially in order to improve the quality of the images on the screen.
These digital type of installations normally use pick-up tubes of the plumbicon or saticon type, with photosensitive elements of very low remanence. The remanence of a photosensor is the property possessed by this device whereby, after receiving a light impulse during one frame (television picture), it gives an electrical signal, that decreases over time, to the following frame (or frames), after the disappearance of this light impulse. In other words, a photosensor retains the light signal received in memory for a certain period of time. High remanence implies a small decrease of the signal with time and, conversely, low remanence implies a big decrease of the signal given by the pick-up device.
A low remanence may be a great disadvantage, especially in radiology installations where it is sought to use the smallest possible dose of X-ray radiation for the patient, the result of which is a decrease in the signal/noise ratio owing to fluctuations of quantal origin, these fluctuation being limited by remanence which constitutes an integration in time.
This is why, in radioscopic installations using the plumbicon or saticon type of pick-up tube, a remanence is artificially introduced by means of a recursive filter.
This filter is generally made by means of a programming of digital processing means.
The inventor has observed that, in radiology installations of this type using television cameras with photosensitive elements of low remanence, the moving images are excessively fuzzy whereas this defect is far less apparent in radiology installations using cameras with vidicon or chalnicon tubes.
On the basis of these observations, the inventor has discovered that the difference in behaviour between low-remanence and high-remanence cameras is derived from the fact that, in tubes of the high-remanence vidicon type, this remanence varies with the level of the signal: the remanence is all the higher as the level of the signal is low. In order words, for strong signals (generally the white ones), the period of retention in memory is generally smaller than for weak signals. This property of the vidicon tube favours the viewing of the image on a television monitor by the human eye. For, in the dark zones of the picture, the quantal noise is all the more troublesome as the signal is weak. Furthermore, the eye is very sensitive to low luminosities. It will therefore be hampered more by fluctuations in the dark zones than by fluctuations in the light zones of the image. Higher remanence in the dark zones than in the light zones is therefore clearly an advantage for the dark zones. For the light zones, low remanence makes it possible, for moving pictures, to limit the streaking or overlapping of these light zones on the dark zones thus reducing the "lined" appearance and, hence, the fuzziness of the image.
Until now, with low-remanence pick-up tubes, an artificial remanence, independent of the signal level, is created, and this artificial remanence is sufficient to reduce the noise, i.e. fluctuations of quantal origin, in the dark zones but is not enough to prevent the fuzziness of moving pictures. In the installation according to the invention, the artificial remanence varies withthe level of the signal. With the tube of the plumbicon or saticon type, a greater artificial remanence will be chosen for the dark zones than for the light zones. For tubes in which the noise is greater in the light zones than in the dark zones, the artificial remanence will vary in reverse to the above phenomenon, depending on the level, namely it will be greater at a high level than at a low level.
The description below will be limited to examples of remanence which are greater at low levels than at high levels.
3. Summary of the Invention
More precisely, the invention pertains to an imaging installation, especially for digital type radiology, comprising a television camera with a pick-up device presenting no remanence or low remanence, with a means such as a recursive filter to produce a remanence of the image signal given by the camera, an installation wherein the remanence varies with the level of the signal.
In the preferred embodiment of the invention, the recursive filter is such that it simulates the remanence of a vidicon tube.